An ink jet recording method has drawn public attention because it is a nonimpact recording process capable of directly recording on an ordinary paper at a high speed, thus providing high image quality while using an apparatus of simple construction. In particular, an electrostatic suction-type ink jet recording method ejects ink by an electrostatic Coulomb's force in response to an electric signal. According to this method, the structure of the recording head is simple. The recording head is designed to have a recording width corresponding to the width of the recording paper. Further, by modulating the pulse width, the dot diameter can be modulated to form an image with a multi-gradation. Electrostatic suction-type ink jet recording methods which have heretofore been proposed can be classified by the structure of recording head into the following groups: single-nozzle processes in which a single nozzle is mechanically scanned during recording; multi-nozzle processes in which a number of nozzles arranged corresponding to pixels are electronically and horizontally scanned during recording; slit jet processes in which a single partition-free slit opening, with in which recording electrodes are arranged corresponding to pixel, is electronically and horizontally scanned during recording; and thermal slit jet processes in which a portion of ink which has been heated and fluidized to have low viscosity corresponding to an image, is drawn by a uniform electric field.
In the electrostatic attracting-type ink jet recording method, a voltage pulse is applied across a recording electrode on the recording head and an opposing electrode positioned on the back side of a recording medium or intermediate recording material such as recording paper, whereby the resulting Coulomb's force causes ink in the recording head to jet towards the recording medium or intermediate recording material to form a dot thereon so that an image is eventually formed. During this process, the ink jets or flies towards the recording medium or intermediate recording material while drawing a thread.
However, the foregoing electrostatic attraction-type ink jet recording method is disadvantageous in that the drawing direction of the flying ink from the recording head onto the recording medium or intermediate recording material is deviated by some actions, making it impossible to ensure that an ink can fly onto a proper position on the recording medium or intermediate recording material to form a dot. This results in a marked deterioration of the quality of the image formed on the recording medium or intermediate recording material.
The inventors made extensive studies of this phenomenon. As a result, it has been found that the factor which has a great effect on the drawing direction of a flying ink is the dielectric constant of the ink and volume resistivity of the ink rather than the dielectric constant of the recording medium or intermediate recording material and/or resistivity thereof. In some detail, the charged condition of a dot formed by an ink which has jetted towards the recording medium or intermediate recording material affects the drawing direction of the subsequently flying ink. Thus, the drawing line of the subsequently flying ink is bent, deviating the position of the subsequently formed dot from the predetermined position. This results in a marked deterioration of the image quality.
The foregoing phenomenon will be further discussed below. Referring first to FIG. 1, when a predetermined voltage pulse is applied with a power supply 4 across a recording electrode 1 in a recording head and an opposing electrode 3 positioned on the back side of a recording medium 2 such as recording paper, an ink 6 in an orifice 5 in the recording head then jets towards the recording medium 2 while drawing a thread to form a dot 7a thereon.
In this process, if the ink 6 has too high a resistivity, the drawing thread is cut, leaving a positive charge generated by electrostatic induction on the dot 7a, a positive charge remains on the dot 7a as shown in FIG. 2. If ink 6 is allowed to fly to form a subsequent dot 7b under these conditions, the drawing thread for the subsequent dot 7b runs against the positive charge on the dot 7a and is bent away from the dot 7a (i.e., is repelled by the dot 7a) to form a dot 7b in a position deviated from the predetermined position away from the dot 7a.
On the other hand, if the ink 6 has too low a resistivity, no positive charge remains on the dot 7a unlike the foregoing case as shown in FIG. 3. However, since the ink 5 has too low a resistivity, the dot 7a induces an electric charge of the same polarity as the opposing electrode 3 positioned on the back side of the recording medium 2 (negative charge in this case) due to the influence of the opposing electrode 3. If an ink 6 is allowed to jet to form a subsequent dot 7b, the drawing thread for the subsequent dot 7b is attracted by the negative charge on the dot 7a and is bent towards the dot 7a to form a subsequent dot 7b in a position deviated from the predetermined position close to the dot 7a.
Thus, the effect of an ink dot on the drawing thread for the subsequent dot may be repulsion or attraction depending on the charged condition of the ink dot. As a result of experiments made by the inventors focusing on the relationship between the relaxation time calculated by multiplying the dielectric constant of the ink by the volume resistivity of the ink and the ink flying period, it was found that there is a region in which an ink dot which has been formed has no substantial effect on the drawing ink thread for the subsequent dot, making it possible to form the subsequent dot in a predetermined position and eventually form an image with a high quality. Thus, the present invention has been worked out.